A hearing device comprising a receiver and a microphone may experience feedback. Feedback is a severe problem. It refers to a process in which a part of the receiver output is picked up by the microphone, amplified by the hearing device processing and sent out by the receiver again. When the hearing device amplification is larger than the attenuation of the feedback path, instability may occur and usually results in feedback whistling, which limits the maximum gain that can be achieved, and thus feedback compromises the comfort of wearing hearing devices.
J. Maxwell and P. Zurek, “reducing acoustic feedback in hearing aids”, IEEE Transactions on speech and audio processing 3 (4), pp 304-323 (1995) proposed an adaptive feedback cancellation (AFC) using an adaptive Finite-Impulse-Response (FIR) filter to model the overall feedback path. This model needs a long filter to cover the major part of the feedback path impulse response and therefore has a slow converge speed and a high computational load.
To address these issues, U.S. Pat. No. 6,072,884 discloses an alternative form of the feedback path model, which represents the feedback path with two parts: a short adaptive FIR filter and a fixed filter (usually an IIR filter). The fixed filter aims at modeling the invariant or slowly-varying portion of the feedback path, whereas the adaptive filter tracks the rapidly-changing part. This model generally yields a shorter adaptive FIR filter, a faster converge speed and a smaller computational load.
However, the way to obtain the coefficients of the fixed filter in practice is to measure the feedback path for each individual user when the hearing aid is fitted to the user by a dispenser or other person trained in fitting the hearing aid to the user, and fit the fixed filter to model the measured response. This not only requires an additional fitting step, but also fails to capture the true invariant part of the feedback path because the feedback path measured by the dispenser already includes some of the variant parts. Thus, the above measured feedback path includes not only the invariant effects but also some variant effects. For example, the fitting of the hearing aid in the ear canal is included in the invariant part but it may be subject to changes when the user yawns or when the hearing aid is re-inserted to the ear.
Therefore, it is an object to provide a hearing device with improved feedback path model.